Portable liquid storage tanks are employed in a wide variety of activities requiring temporary liquid storage. Such activities include contaminated ground water treatment, liquid waste reduction, chemical cleaning of heat exchangers, chemical test projects, boiler cleaning, ship and barge cleaning, sludge pond clean-up and removal, oil and water separation, tank cleaning and maintenance, underground tank removal, repair and replacement, pipeline cleaning and hydrostatic testing, and use as temporary catch basins for rain water. Various types of portable liquid storage tanks are in use, including polyethylene tanks, vapor-tight steel tanks, and open and closed top steel tanks. Portable tanks typically are transported by trailer, or are mounted on dollies and equipped with hitches to permit towing.
A typical known portable steel tank, denoted 10 in FIGS. 1 and 2, is generally rectangular viewed from above. Tank 10 has a central section 12 and two end sections 14 having poop decks 16 which are vertically offset from the bottom 18 of central section 12 to accommodate a dolly 20 and a trailer hitch 22 at either end, and which are inclined toward central section 12 to facilitate liquid drainage. Tank 10 includes frame 24 comprising a plurality of members 26 (frame 24 is here shown in simplified form, with a reduced number of frame members 26, for illustrative convenience). Members 26 of frame 24 can also include internal bracing members (not shown), as well as bracing members extending across the top opening of the tank (partially shown in FIG. 2). Frame 24 is typically constructed using post-and-beam fabrication with pinned joints 28 connecting members 26. Walls 30 comprise a plurality of plates 32 attached to the outside of frame 24 and define storage volume 34 within tank 10. Outlet valve 36 in the bottom of tank 10 allows drainage of liquid from the tank 10.
Another known portable liquid storage tank is similar to the preceding tank, but employs corrugated steel walls, typically with corrugations of from one to four inches in depth.
Known portable steel tanks suffer from a number of disadvantages. For example, the intersections at which the internal frame members 26 meet plates 32 forming the floor of central section 12 create traps 38 (shown in FIG. 2) within storage volume 34, blocking the fluid flow path to outlet valve 36 and making thus it difficult to drain all stored liquid and to clean the tank after the liquid is removed. Failure to remove all residual liquids results in contamination of subsequent stored liquids. Tank failure can result due to excessive deflection or flexure of tank walls 30, particularly as a consequence of excessive vibration during transport of the tank which can cause separation of wall plates 32. Furthermore, contact of corrosive liquids, such as acidic sludges, drilling muds, and other toxic wastes damages frame members 26 and wall plates 32 and can ultimately result in tank failure. Even without tank failure, replacement of frame members 26 and wall plates 32 damaged by contact with corrosive liquids is difficult, time consuming and expensive.
Tanks having corrugated steel walls also suffer from the disadvantage that the walls themselves are susceptible to attack from corrosive liquids. These tanks are even more difficult and costly to clean than tanks having flat wall sections. The cleaning process is labor-intensive and potentially dangerous to workers who are required to physically enter the tank.
A need has existed for a portable liquid storage tank which does not trap liquids and thus is easy to drain completely and easy to clean after use. It would further be desirable to provide a portable liquid storage tank having a structure which is less susceptible to attack from corrosive liquids stored therein.
A need has also existed for a portable liquid storage tank which can be transported with reduced risk of tank wall failure.